1. Field of the Invention
This invention relates to article theft detection and more particularly it concerns novel apparatus and methods for electronically detecting the passage of protected articles through a interrogation zone such as a "wrap desk" counter or the exit from a store or other protected area.
2. Description of the Prior Art
The present invention constitutes improvements over the stolen merchandise detection method and apparatus described in U.S. Pat. No. 3,740,742 to Thomas F. Thompson and Joseph W. Griffith. That patent describes apparatus for detecting the passage of a resonant electronic circuit through an aisle in a store through which customers must pass. Plates or coils are provided along the aisle and then are energized with pulses to produce sharp electrostatic or electromagnetic pulses in the aisle. These pulses cause resonant electrical circuits, attached to the protected articles carried through the aisle, to resonate for a duration following each pulse. A receiver is provided to detect the resultant radiation from the resonant circuits and the receiver is gated to detect signals only after the energizing pulse has terminated.
Other apparatus which detect resonant electrical circuits by generating pulses and monitor the resulting radiation from the resonanting circuits are shown and described in U.S. Pat. Nos. 2,812,427, 2,899,546, 2,958,781, 3,117,277 and 3,373,425. These patents, however, are concerned principally with long distance signal transmission and they have nothing to do with detection of stolen articles.
A disadvantage of the above described theft detection method and apparatus, and of the devices of the other patents referred to above, if they were applied to theft detection, is that they are, or would be, susceptible to false alarms caused by radiation from other sources in the vicinity of the apparatus. The signal emanating from the resonant circuit is of extremely low amplitude and it is easily overpowered by radiation from nearby electrical equipment such as lights, motors or switches or even from nearby theft detection equipment; and in some cases from ringing or transients within the detection equipment itself. It has been proposed to reduce the interference of other radiation sources by making the monitoring apparatus sensitive only to the specific resonant frequency of the circuits on the protected articles. However, it is difficult in practice to make those circuits each resonate at precisely the same frequency. Moreover, the circuits are sometimes subject to detuning, as when they are placed next to a piece of metal or next to another resonant circuit. Also, this does not protect against transient responses or continuous electrical noise from various sources other than the resonant circuits being detected and which produce signals at the resonant frequency of the resonant circuit being detected.